Radiating structural body of electronic part and radiating sheet used for the radiating structural body

ABSTRACT

A heat dissipating structure for a heat generating electronic component is characterized by comprising a heat dissipating sheet including a metal sheet and a heat conductive member having adhesion stacked thereon between the heat generating electronic component and a heat dissipating member, wherein the metal sheet is connected to the heat generating electronic component, and the heat conductive member having adhesion is connected to the heat dissipating member.

TECHNICAL FIELD

This invention relates to a heat dissipating structure which caneffectively conduct the heat generated by heat generating electroniccomponents such as transistors and computer CPUs to heat dissipatingmembers such as heat sinks, and a heat dissipating sheet used therein.

BACKGROUND ART

The recent drive for higher integration and operating speeds in CPUs,driver ICs, memories and other LSIs used in electronic equipment such aspersonal computers, digital video disks and mobile phones has increasedthe power consumption. Concomitantly the heat release has increased,which can cause malfunction of electronic equipment or damages toelectronic components. It becomes very important to address the heatdissipation.

In prior art electronic equipment, heat sinks in the form of plates madeof brass and other high thermal conductivity metals are used in order tosuppress a temperature rise of electronic components during operation.The heat sinks conduct the heat generated by the electronic componentsand release the heat from their surface by means of a temperaturedifference from the ambient air.

For effective heat transfer from electronic components to heat sinks,the heat sink must be placed in intimate contact with the electroniccomponent. Because of height differences among various electroniccomponents and tolerance in the assembly process, a flexible heatconductive sheet or a heat conductive grease is interposed between theelectronic component and the heat sink so that the heat transfer fromthe electronic component to the heat sink is established through theheat conductive sheet or heat conductive grease. The heat conductivesheet used is typically a heat conductive sheet made of heat conductivesilicone rubber or the like (i.e., heat conductive silicone rubbersheet), and the heat conductive grease used is typically a heatconductive silicone grease.

The conventionally used heat conductive silicone rubber sheet, however,has a substantial contact thermal resistance at the interface with anelectronic component, which imposes a certain limit to the heat transferperformance. This is a serious problem when electronic components ofhigh-frequency drive with substantial heat generation such as CPUs areto be cooled. It is desired to reduce the interfacial contact thermalresistance.

On the other hand, heat conductive grease has a substantially negligiblelevel of interfacial contact thermal resistance because of itsliquid-like nature and exerts good heat transfer performance, butsuffers from the problems that a special equipment such as dispenser isneeded and its recovery entails an inefficient operation.

To overcome some of the above-described problems, a phase change heatdissipating sheet (simply phase change sheet) was proposed which is asolid sheet at normal room temperature, but softens by the heatgenerated by a electronic component during operation, to reduce theinterfacial contact thermal resistance to a negligible level. Thefollowing phase change sheets were proposed in the prior art. U.S. Pat.No. 4,466,483 discloses a non-metallic sheet having phase change waxlayers formed on opposite surfaces thereof. Also, U.S. Pat. No.5,904,796 discloses a metal foil having a phase change paraffin orpetroleum jelly formed on one surface thereof and an adhesive formed onthe other surface thereof. JP-A 2000-509209 discloses a phase changesheet characterized by comprising an acrylic pressure-sensitiveadhesive, a wax and a heat conductive filler, with an intermediate layerof network structure, film or the like being excluded.

On the other hand, the heat dissipating sheet is, on use, interposedbetween a heat dissipating member such as a heat sink and a heatgenerating electronic component such as CPU. In the recent industry, asituation has arisen where the process of producing personal computercomponents and the process of assembling such components to completepersonal computers are separately performed at different locations overthe world. It thus becomes a routine to attach a heat dissipating sheetto a heat sink at one location and transport the assembly to anotherlocation where it is mounted to CPU.

When the heat dissipating sheet-attached heat sink assembly is to beinspected for quality, the assembly must be mounted to a model CPU andsubjected to a burn-in test or thermal cycling test under the same stateas actual mounting. In the test, the phase change sheet works wellbecause it melts at a temperature above the melting point and comes inclose contact with the heat sink and the CPU. However, the phase changesheet can fail when the heat sink is separated from the CPU after thetest. Even the electronic component and the heat sink can also failbecause of strong adhesive bonds. In the event of failure of the phasechange sheet, not only a new phase change sheet must be attached again,but a test must be done again to inspect whether the new phase changesheet meets the desired quality, which is unreasonable. After assemblyto a personal computer, a burn-in test or thermal cycling test issometimes done, giving rise to the same problem.

Moreover, U.S. Pat. No. 5,550,326 disclose a heat dissipator or heatdissipating structure in which an adhesive heat conductive pad layer isformed on one surface of a metal foil and which is used such that thesurface of the adhesive heat conductive pad layer is in close contactwith the electronic component side and the surface of the metal foil isin close contact with a heat dissipating member. This structure has theproblem that since the adhesive heat conductive pad is in close contactwith the electronic component, an attempt to peel the adhesive heatconductive pad from the electronic component can result in failure ofthe adhesive heat conductive pad and even failure of the electroniccomponent.

The present invention intends to improve the above-discussed situationand its object is to provide a heat dissipating structure for a heatgenerating electronic component, which can effectively conduct the heatgenerated by the electronic component to a heat dissipating member suchas a heat sink, and which even after a burn-in test or thermal cyclingtest to be performed for quality inspection, allows the electroniccomponent and the heat dissipating member to be separated withoutstructural failure of a heat dissipating sheet or failure of theelectronic component, as well as a heat dissipating sheet used therein.

DISCLOSURE OF THE INVENTION

To attain the above object, the present invention provides a heatdissipating structure for a heat generating electronic component,characterized by comprising a heat dissipating sheet including a metalsheet and a heat conductive member having adhesion stacked thereonbetween the heat generating electronic component and a heat dissipatingmember, wherein the metal sheet is connected to the heat generatingelectronic component, and the heat conductive member having adhesion isconnected to the heat dissipating member. The present invention alsoprovides a heat dissipating sheet interposed between a heat generatingelectronic component and a heat dissipating member, characterized bycomprising a metal sheet to be disposed on the side of the heatgenerating electronic component and a heat conductive member havingadhesion stacked on the metal sheet and to be disposed on the side ofthe heat dissipating member, wherein the heat conductive member havingadhesion is formed of a heat conductive composition comprising at leastone resin component selected from among siloxane polymers, acrylicpolymers and polyolefin polymers, and a heat conductive filler, thecomposition being able to phase change or soften by the heat from theelectronic component during operation.

According to the invention, heat can be effectively conducted from theheat generating electronic component to the heat dissipating member,typically a heat sink. Even after a burn-in test or thermal cyclingtest, the electronic component or heat dissipating member can be removedfrom the heat dissipating sheet without inconvenient problems includingstructural failure of the heat dissipating sheet and failure of theelectronic component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a heat dissipating sheet accordingto one embodiment of the invention.

FIG. 2 is a cross-sectional view of a heat dissipating structureaccording to one embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Now the present invention is described in further detail.

The heat dissipating sheet of the invention is to be interposed betweena heat generating electronic component and a heat dissipating member andis characterized by comprising a metal sheet to be disposed on the sideof the heat generating electronic component and a heat conductive memberhaving adhesion stacked on the metal sheet and to be disposed on theside of the heat dissipating member, wherein the heat conductive memberhaving adhesion is formed of a heat conductive composition comprising atleast one resin component selected from among siloxane polymers, acrylicpolymers and polyolefin polymers, and a heat conductive filler, thecomposition being able to phase change or soften by the heat from theelectronic component during operation.

The preferred metal sheet used herein is an aluminum sheet, coppersheet, stainless steel sheet, tungsten sheet, gold sheet or the like,with the aluminum and copper sheets being more preferred for low cost.The metal sheet preferably has a thickness in the range of 3 to 200 μm,more preferably in the range of 5 to 75 μm. With a thickness of lessthan 3 μm, strength may be short. A thickness in excess of 200 μm mayadversely affect the flexibility of the heat dissipating sheet,detracting from close contact with the electronic component.

The heat conductive member used herein may be formed of a clay-like heatconductive composition having pressure-sensitive adhesion, typical ofwhich is an acrylic base pressure-sensitive adhesive, siloxane basepressure-sensitive adhesive or olefin base pressure-sensitive adhesivein which a heat conductive filler is compounded.

Preferably the clay-like heat conductive composition has a plasticity at25° C. in the range of 100 to 1,000. With a plasticity of less than 100,the heat conductive member may have low strength and become lessconvenient to handle. With a plasticity of more than 1,000, the heatdissipating sheet may lack flexibility, giving rise to a problem to theclose contact with the heat dissipating member.

In a preferred embodiment, the adhesive heat conductive member containsa phase transition material or thermosoftening material, because inresponse to heat generation of the electronic component duringoperation, the heat conductive member changes its phase from the solidto a liquid or fluid/semifluid or thermally softens for reducing thecontact thermal resistance between it and the heat dissipating member.The phase transition or thermosoftening materials used herein includeparaffin waxes, α-olefins, silicone resins, and fluoro-resins. Thesephase transition or thermosoftening materials preferably have anendothermic peak associated with phase change or thermal softening asmeasured by a differential scanning calorimeter (DSC) in the range of 35to 120° C., more preferably 40 to 100° C., especially the operatingtemperature range of electronic components of 50 to 80° C.

The heat conductive fillers used in the heat conductive member includemetal powders such as iron, aluminum, nickel, silver, and gold,inorganic oxide powders such as silicon oxide, aluminum oxide, zincoxide, iron oxide, and magnesium oxide, and inorganic nitride powderssuch as aluminum nitride and boron nitride. The heat conductive fillermay have an average particle size in the range of 0.1 to 30 μm, with themaximum particle size being preferably up to 100 μm.

It is understood that the amount of the phase transition orthermosoftening material compounded and the amount of the heatconductive filler compounded in the acrylic, siloxane or olefin basepressure-sensitive adhesive may be determined as appropriate. Preferablythe amount of phase transition or thermosoftening material is 10 to1,000 parts by weight, especially 50 to 500 parts by weight per 100parts by weight of the adhesive, and the amount of heat conductivefiller is 0 to 3,000 parts by weight, especially 100 to 2,000 parts byweight per 100 parts by weight of the adhesive.

With respect to the thickness of the adhesive heat conductive member, athinner member is advantageous because the thermal resistance becomeslower. However, it is substantially impossible to reduce the thicknessbelow the maximum particle size of the heat conductive filler. Thus thethickness preferably ranges from a value equal to the maximum particlesize of the heat conductive filler to an approximate 5 fold, andtypically from 0.1 to 1,000 μm, especially from 1 to 500 μm.

The method of preparing the heat dissipating sheet comprising a metalsheet and a heat conductive member formed on one surface thereof is, forexample, by dissolving the heat conductive composition in an organicsolvent and applying the resulting slurry by any well-known coating orspraying technique. FIG. 1 illustrates the construction of the heatdissipating sheet of the invention. In FIG. 1, 1 denotes the heatdissipating sheet, 1-a denotes a heat conductive member, and 1-b denotesa metal sheet.

In another aspect, the present invention provides a heat dissipatingstructure for a heat generating electronic component, comprising theabove-described heat dissipating sheet including a metal sheet and anadhesive heat conductive member stacked thereon between the heatgenerating electronic component and a heat dissipating member, the metalsheet being connected to the heat generating electronic component, andthe adhesive heat conductive member being connected to the heatdissipating member. As shown in FIG. 2, this structure is constructed ashaving a heat dissipating sheet 1 interposed between an electroniccomponent 2 and a heat dissipating member 3. The heat conductive member1-a is connected to the heat dissipating member 3, and the metal sheet 1b connected to the electronic component 2. With these connections, evenafter a burn-in test or thermal cycling test to be performed for qualityinspection, the electronic component and the heat dissipating member canbe removed without structural failure of the heat dissipating sheet.Since the heat conductive member in this heat dissipating structure hasan adequate degree of pressure-sensitive adhesion, the heat dissipatingsheet can be removed as being attached to the heat dissipating member.Moreover, since the adhesive heat conductive member is reinforced withthe metal sheet, the heat dissipating sheet can be removed from the heatdissipating member without structural failure of the sheet, ensuringeasy repair.

EXAMPLE

Examples are given below for illustrating the invention, but theinvention is not limited to the examples.

Examples 1–6

20 parts by weight of xylene was added to 100 parts by weight of a heatconductive composition of the formulation shown in Table 1 to form aslurry, which was applied onto a metal sheet by means of a bar coater,and dried at 80° C. for 20 minutes, obtaining a heat dissipating sheetof 0.1 mm thick.

The heat dissipating sheet was measured for physical properties by thefollowing measurement methods, with the results shown in Table 1.

It is seen from these results that the heat dissipating structure andheat dissipating sheet of the invention are very useful.

Measurement Methods

1) Plasticity Measurement:

Measured by the plasticity test of JIS K-6249

2) Thermal Conductivity Measurement:

Measured by a thermal conductivity meter QTM-500 (trade name, KyotoDenki Co., Ltd.)

3) Thermal Resistance Measurement:

A sample of 0.5 mm thick punched into a TO-3 transistor shape wasinterposed between a transistor 2SD923 (trade name, Fuji Electric Co.,Ltd.) and a heat sink FBA-150-PS (trade name, OS Co., Ltd.), and acompression load of 1000 gf/cm² was applied. The heat sink was placed ina thermostat water tank and held at 60° C.

Then a power of 10 V and 3 A was fed to the transistor. After 5 minutes,the temperature (T1) of the transistor and the temperature (T2) of theheat sink were measured using thermocouples embedded in the transistorand heat sink. The thermal resistance Rs (in ° C./W) of the sample wascomputed according to the equation: Rs=(T1−T2)/30.

4) Adhesive Strength Measurement:

Measured by the adhesive tape test of JIS Z-0237

5) Thermosoftening Temperature:

Measured from an endothermic peak by DSC

6) Repair:

At the end of the above thermal resistance measurement, the transistorand the heat sink were separated apart. The heat dissipating sheet wasrated OK (repairable) when it was kept connected to the heat sink sidewith its shape retained.

TABLE 1 Compounding amount (pbw) Example 1 Example 2 Example 3 Example 4Example 5 Example 6 Acrylic adhesive 100 0 0 50 0 0 Silicone adhesive 0100 0 0 50 0 Olefin adhesive 0 0 100 0 0 50 α-olefin 0 0 0 0 0 50Paraffin wax 0 0 0 50 0 0 Silicone resin 0 0 0 0 50 0 Heat conductivefiller 1 1400 1400 1400 0 0 0 Heat conductive filler 2 0 0 0 800 800 800Metal sheet Al foil Al foil Al foil Cu foil Cu foil Cu foil 0.05 mm 0.05mm 0.05 mm 0.018 mm 0.018 mm 0.018 mm Thickness, mm 0.1 0.1 0.1 0.1 0.10.1 Thermal conductivity, W/mK 3.2 3.1 3.0 4.1 4.0 3.9 Thermalresistance, ° C./W 0.06 0.06 0.07 0.03 0.03 0.03 Thermosoftening temp.,° C. — — — 55 59 65 Adhesive strength, N/cm 0.60 0.45 0.17 0.13 0.150.10 Plasticity 300 310 400 500 600 700 Repair OK OK OK OK OK OK

Materials Used in Examples

-   Acrylic pressure-sensitive adhesive: SK-DYNE 1345 (trade name, Soken    Chemical & Engineering Co., Ltd.)-   Silicone pressure-sensitive adhesive: KR130 (trade name, Shin-Etsu    Chemical Co., Ltd.)-   Olefin pressure-sensitive adhesive: Lucant HC3000X (trade name,    Mitsui Chemical Co., Ltd.)-   α-olefin: DIALEN 30 (trade name, Mitsubishi Chemical Corp.)-   Paraffin wax: 130 (trade name, Nippon Seiro Co., Ltd.)-   Silicone resin: X-40-9800-59 (trade name, Shin-Etsu Chemical Co.,    Ltd.)-   Heat conductive filler 1: alumina AO-41R (trade name, Admatechs Co.,    Ltd.)-   Heat conductive filler 2: silver powder Ag-E-100 (trade name, Fukuda    Metal Foil/Powder Industry Co., Ltd.)-   Aluminum foil, copper foil: made by Fukuda Metal Foil/Powder    Industry Co., Ltd.

The heat dissipating sheet for a heat generating electronic componentaccording to the present invention can effectively conduct the heatgenerated by the electronic component to a heat dissipating member suchas a heat sink, and even after a burn-in test or thermal cycling test tobe performed for quality inspection, allows the electronic component andthe heat dissipating member to be separated without structural failureof the heat dissipating sheet or failure of the electronic component.

1. A heat dissipating structure for a heat generating electroniccomponent, comprising a heat dissipating sheet including a metal sheetand a heat conductive member having adhesion, stacked on said metalsheet, between the heat generating electronic component and a heatdissipating member, wherein the metal sheet is connected to the heatgenerating electronic component, and the heat conductive member havingadhesion is connected to the heat dissipating member, wherein said heatconductive member having adhesion is formed of a heat conductivecomposition comprising at least one resin component selected from thegroup consisting of pressure sensitive adhesive siloxane polymers, aheat conductive filler, and a phase transition material orthermosoftening material selected from the group consisting of siliconeresins, the composition having a plasticity at 25° C. in the range of100 to 1,000, as measured by JIS K-6249, and being able to phase changeor soften by the heat from the electronic component during operation. 2.The heat dissipating structure of claim 1 wherein the metal sheet is analuminum sheet or copper sheet.
 3. A heat dissipating sheet adapted tobe interposed between a heat generating electronic component and a heatdissipating member, comprising a metal sheet adapted to be disposed onthe side of the heat generating electronic component and a heatconductive member having adhesion, stacked on the metal sheet andadapted to be disposed on the side of the heat dissipating member,wherein the heat conductive member having adhesion is formed of a heatconductive composition comprising at least one resin component selectedfrom the group consisting of pressure sensitive adhesive siloxanepolymers, a heat conductive filler, and a phase transition material orthermosoftening material selected from the group consisting of siliconeresins, the composition having a plasticity at 25° C. in the range of100 to 1,000, as measured by JIS K-6249, and being able to phase changeor soften by the heat from the electronic component during operation. 4.The heat dissipating structure of claim 1, wherein the phase transitionor thermosoftening material is present in an amount of 10 to 1,000 partsby weight, per 100 parts by weight of the resin component, and the heatconductive filler is present in an amount of 100 to 3,000 parts byweight, per 100 parts by weight of the resin component.
 5. The heatdissipating structure of claim 1, wherein the phase transition orthermosoftening material is present in an amount of 50 to 500 parts byweight, per 100 parts by weight of the resin component, and the heatconductive filler is present in an amount of 100 to 2,000 parts byweight, per 100 parts by weight of the resin component.
 6. The heatdissipating structure of claim 1, wherein the metal sheet has athickness of 3 to 200 μm.
 7. The heat dissipating structure of claim 1,wherein the metal sheet has a thickness of 5 to 75 μm.
 8. The heatdissipating structure of claim 1, wherein the phase transition orthermosoftening material has an endothermic peak associated with phasechange or thermosoftening as measured by a differential scanningcalorimeter in the range of 35 to 120° C.
 9. The heat dissipatingstructure of claim 8, wherein the phase transition or thermosofteningmaterial has an endothermic peak associated with phase change orthermosoftening as measured by a differential scanning calorimeter inthe range of 40 to 100° C.
 10. The heat dissipating structure of claim9, wherein the phase transition or thermosoftening material has anendothermic peak associated with phase change or thermosoftening asmeasured by a differential scanning calorimeter in the range of 50 to80° C.
 11. The heat dissipating structure of claim 1, wherein the heatconductive filler is present and has an average particle size in therange of 0.1 to 30 μm, and a maximum particle size of up to 100 μm. 12.The heat dissipating structure of claim 11, wherein the heat conductivemember has a thickness ranging from a value equal to the maximumparticle size of the heat conductive filler to an approximate 5 fold.13. The heat dissipating structure of claim 1, wherein the heatconductive member has a thickness of from 0.1 to 1,000 μm.